Methods and Apparatus for Generating Strongly-Ionized Plasmas with Ionizational Instabilities

ABSTRACT

Methods and apparatus for generating strongly-ionized plasmas are disclosed. A strongly-ionized plasma generator according to one embodiment includes a chamber for confining a feed gas. An anode and a cathode assembly are positioned inside the chamber. A pulsed power supply is electrically connected between the anode and the cathode assembly. The pulsed power supply generates a multi-stage voltage pulse that includes a low-power stage with a first peak voltage having a magnitude and a rise time that is sufficient to generate a weakly-ionized plasma from the feed gas. The multi-stage voltage pulse also includes a transient stage with a second peak voltage having a magnitude and a rise time that is sufficient to shift an electron energy distribution in the weakly-ionized plasma to higher energies that increase an ionization rate which results in a rapid increase in electron density and a formation of a strongly-ionized plasma.

BACKGROUND OF INVENTION

A plasma can be created in a chamber by igniting a direct current (DC)electrical discharge between two electrodes in the presence of a feedgas. The electrical discharge generates electrons in the feed gas thationize atoms thereby creating the plasma. The electrons in the plasmaprovide a path for an electric current to pass through the plasma. Theenergy supplied to the plasma must be relatively high for applications,such as magnetron plasma sputtering. Applying high electrical currentsthrough a plasma can result in overheating the electrodes as well asoverheating the work piece in the chamber. Complex cooling mechanismscan be used to cool the electrodes and the work piece. However, thecooling can cause temperature gradients in the chamber. Thesetemperature gradients can cause non-uniformities in the plasma densitywhich can cause non-uniform plasma process.

Temperature gradients can be reduced by pulsing DC power to theelectrodes. Pulsing the DC power can allow the use of lower averagepower. This results in a lower temperature plasma process. However,pulsed DC power systems are prone to arcing at plasma ignition andplasma termination, especially when working with high-power pulses.Arcing can result in the release of undesirable particles in the chamberthat can contaminate the work piece.

Plasma density in known plasma systems is typically increased byincreasing the electrode voltage. The increased electrode voltageincreases the discharge current and thus the plasma density. However,the electrode voltage is limited in many applications because highelectrode voltages can effect the properties of films being deposited oretched. In addition, high electrode voltages can also cause arcing whichcan damage the electrode and contaminate the work piece.

BRIEF DESCRIPTION OF DRAWINGS

This invention is described with particularity in the detaileddescription and claims. The above and further advantages of thisinvention may be better understood by referring to the followingdescription in conjunction with the accompanying drawings, in which likenumerals indicate like structural elements and features in variousfigures. The drawings are not necessarily to scale, emphasis insteadbeing placed upon illustrating the principles of the invention.

FIG. 1 illustrates a cross-sectional view of a plasma sputteringapparatus having a pulsed direct current (DC) power supply according toone embodiment of the invention.

FIG. 2 is measured data of discharge voltage as a function of dischargecurrent for a prior art low-current plasma and a high-current plasmaaccording to the present invention.

FIG. 3 is measured data of a particular voltage pulse generated by thepulsed power supply of FIG. 1 operating in a low-power voltage mode.

FIG. 4 is measured data of a multi-stage voltage pulse that is generatedby the pulsed power supply of FIG. 1 that creates a strongly-ionizedplasma according to the present invention.

FIG. 5A-FIG. 5C are measured data of other illustrative multi-stagevoltage pulses generated by the pulsed power supply of FIG. 1.

FIG. 6A and FIG. 6B are measured data of multi-stage voltage pulsesgenerated by the pulsed power supply of FIG. 1 that illustrate theeffect of pulse duration in the transient stage of the pulse on theplasma discharge current.

FIG. 7A and FIG. 7B are measured data of multi-stage voltage pulsesgenerated by the pulsed power supply of FIG. 1 that show the effect ofthe pulsed power supply operating mode on the plasma discharge current.

FIG. 8 is measured data for an exemplary single-stage voltage pulsegenerated by the pulsed power supply of FIG. 1 that produces ahigh-density plasma according to the invention that is useful forhigh-deposition rate sputtering.

FIG. 9 illustrates a cross-sectional view of a plasma sputteringapparatus having a pulsed direct current (DC) power supply according toanother embodiment of the invention.

FIG. 10 illustrates a schematic diagram of a pulsed power supply thatcan generate multi-step voltage pulses according to the presentinvention.

FIG. 11 illustrates a schematic diagram of a pulsed power supply havinga magnetic compression network for supplying high-power pulses.

FIG. 12 illustrates a schematic diagram of a pulsed power supply havinga Blumlein generator for supplying high-power pulses.

FIG. 13 illustrates a schematic diagram of a pulsed power supply havinga pulse cascade generator for supplying high-power pulses.

DETAILED DESCRIPTION

FIG. 1 illustrates a cross-sectional view of a plasma sputteringapparatus 100 having a pulsed direct current (DC) power supply 102according to one embodiment of the invention. The plasma sputteringapparatus 100 includes a vacuum chamber 104 for containing a plasma. Thevacuum chamber 104 can be coupled to ground 105. The vacuum chamber 104is positioned in fluid communication with a vacuum pump 106 that is usedto evacuate the vacuum chamber 104 to high vacuum. The pressure insidethe vacuum chamber 104 is generally less than 10⁻¹ Torr for most plasmaoperating conditions. A process or feed gas 108 is introduced into thevacuum chamber 104 through a gas inlet 112 from a feed gas source 110,such as an argon gas source. The flow of the feed gas is controlled by avalve 114. In some embodiments, the gas source is an excited atom ormetastable atom source.

The plasma sputtering apparatus 100 also includes a cathode assembly116. The cathode assembly 116 shown in FIG. 1 is formed in the shape ofa circular disk, but can be formed in other shapes. In some embodiments,the cathode assembly 116 includes a target 118 for sputtering. Thecathode assembly 116 is electrically connected to a first terminal 120of the pulsed power supply 102 with an electrical transmission line 122.

A ring-shaped anode 124 is positioned in the vacuum chamber 104proximate to the cathode assembly 116. The anode 124 is electricallyconnected to ground 105. A second terminal 125 of the pulsed powersupply 102 is also electrically connected to ground 105. In otherembodiments, the anode 124 is electrically connected to the secondterminal 125 of the pulsed power supply 102 which is not at groundpotential.

A housing 126 surrounds the cathode assembly 116. The anode 124 can beintegrated with or electrically connected to the housing 126. The outeredge 127 of the cathode assembly 116 is electrically isolated from thehousing 126 insulators 128. The gap 129 between the outer edge 127 ofthe cathode assembly 116 and the housing 126 can be an air gap or caninclude a dielectric material.

In some embodiments, the plasma sputtering apparatus 100 includes amagnet assembly 130 that generates a etic field 132 proximate to thetarget 118. The magnetic field 132 is less parallel to the surface ofthe cathode assembly 116 at the poles of the magnets in the magnetassembly 130 and more parallel to the surface of the cathode assembly116 in the region 134 between the poles of the magnets in the magneticassembly 130. The magnetic field 132 is shaped to trap and concentratesecondary electrons emitted from the target 118 that are proximate tothe target surface 133. The magnet assembly can consist of rotatingmagnets.

The magnetic field 132 increases the density of electrons and therefore,increases the plasma density in the region 134 that is proximate to thetarget surface 133. The magnetic field 132 can also induce an electronHall current 135 that is formed by the crossed electric and magneticfields. The strength of the electron Hall current 135 depends, at leastin part, on the density of the plasma and the strength of the crossedelectric and magnetic fields.

The plasma sputtering apparatus 100 also includes a substrate support136 that holds a substrate 138 or other work piece for plasmaprocessing. In some embodiments, the substrate support 136 is biasedwith a RF field. In these embodiments, the substrate support 136 iselectrically connected to an output 140 of a RF power supply 142 with anelectrical transmission line 144. A matching network (not shown) may beused to coupled the RF power supply 142 to the substrate support 136. Insome embodiments, a temperature controller 148 is thermally coupled tothe substrate support 136. The temperature controller 148 regulates thetemperature of the substrate 138.

In some embodiments, the plasma sputtering apparatus 100 includes anenergy storage device 147 that provides a source of energy that can becontrollably released into the plasma. The energy storage device 147 iselectrically coupled to the cathode assembly 116. In one embodiment, theenergy storage device 147 includes a capacitor bank.

In operation, the vacuum pump 106 evacuates the chamber 104 to thedesired operating pressure. The feed gas source 110 injects feed gas 108into the chamber 104 through the gas inlet 112. The pulsed power supply102 applies voltage pulses to the cathode assembly 116 that cause anelectric field 149 to develop between the target 118 and the anode 124.The magnitude, duration and rise time of the initial voltage pulse arechosen such that the resulting electric field 149 ionizes the feed gas108, thus igniting the plasma in the chamber 104.

In one embodiment, ignition of the plasma is enhanced by one or moremethods described in co-pending U.S. patent application Ser. No.10/065,277, entitled High-Power Pulsed Magnetron Sputtering, andco-pending U.S. patent application Ser. No. 10/065,629, entitled Methodsand Apparatus for Generating High-Density Plasma which are assigned tothe present assignee. The entire disclosures of U.S. patent applicationSer. No. 10/065,277 and U.S. patent application Ser. No. 10/065,629 areincorporated herein by reference. U.S. patent application Ser. No.10/065,629 describes a method of accelerating the ignition of the plasmaby increasing the feed gas pressure for a short period of time and/orflowing feed gas directly through a gap between an anode and a cathodeassembly. In addition, U.S. patent application Ser. No. 10/065,277describes a method of using pre-ionization electrodes to accelerate theignition of the plasma.

The characteristics of the voltage pulses generated by the pulsed powersupply 102 and the resulting plasmas are discussed in connection withthe following figures. The pulsed power supply 102 can include circuitrythat minimizes or eliminates the probability of arcing in the chamber104. Arcing is generally undesirable because it can damage the anode 124and cathode assembly 116 and can contaminate the wafer or work piecebeing processed. In one embodiment, the circuitry of the pulse supply102 limits the plasma discharge current up to a certain level, and ifthis limit is exceeded, the voltage generated by the power supply 102drops for a certain period of time.

The plasma is maintained by electrons generated by the electric field149 and also by secondary electron emission from the target 118. Inembodiments including the magnet assembly 130, the magnetic field 132 isgenerated proximate to the target surface 133. The magnetic field 132confines the primary and secondary electrons in a region 134 therebyconcentrating the plasma in the region 134. The magnetic field 132 alsoinduces the electron Hall current 135 proximate to the target surface133 that further confines the plasma in the region 134.

In one embodiment, the magnet assembly 130 includes an electromagnet inaddition to a permanent magnet. A magnet power supply (not shown) iselectrically connected to the magnetic assembly 130. The magnet powersupply can generate a constant current that generates a constantmagnetic filed. Alternatively, the magnet power supply can generate apulse that produces a pulsed magnetic field that creates an increase inelectron Hall current 135 proximate to the target surface 133 thatfurther confines the plasma in the region 134. In one embodiment, thepulsing of the magnetic field is synchronized with the pulsing theelectric field in the plasma discharge in order to increase the densityof the plasma. The sudden increase in the electron Hall current 135 maycreate a transient non-steady state plasma.

Ions in the plasma bombard the target surface 133 because the target 118is negatively biased. The impact caused by the ions bombarding thetarget surface 133 dislodges or sputters material from the target 118.The sputtering rate generally increases as the density of the plasmaincreases.

The RF power supply 142 can apply a negative RF bias voltage to thesubstrate 138 that attracts positively ionized sputtered material to thesubstrate 138. The sputtered material forms a film of target material onthe substrate 138. The magnitude of the RF bias voltage on the substrate138 can be chosen to optimize parameters, such as sputtering rate andadhesion of the sputtered film to the substrate 138. The magnitude ofthe RF bias voltage on the substrate 138 can also be chosen to minimizedamage to the substrate 138. In embodiments including the temperaturecontroller 148, the temperature of the substrate 138 can be regulated bythe temperature controller 148 in order to avoid overheating thesubstrate 138.

Although FIG. 1 illustrates a cross-sectional view of a plasmasputtering apparatus 100, it will be clear to skilled artisans that theprinciples of the present invention can be used in many other systems,such as plasma etching systems, hollow cathode magnetrons, ion beamgenerators, plasma-enhanced chemical vapor deposition (CVD) systems,plasma accelerators, plasma rocket thrusters, plasma traps, and anyplasma system that uses crossed electric and magnetic fields.

FIG. 2 is measured data 150 of discharge voltage as a function ofdischarge current for a prior art low-current plasma and a high-currentplasma according to the present invention. Current-voltagecharacteristic 152 represents measured data for discharge voltage as afunction of discharge current for a plasma generated in a typicalcommercial magnetron plasma system with a commercially available DCpower supply. The actual magnetron plasma system used to obtain thecurrent-voltage characteristics 152 was a standard magnetron with a 10cm diameter copper sputtering target. Similar results have been observedfor a NiV sputtering target. Argon was used as the feed gas and theoperating pressure was about 1 mTorr. The current-voltage characteristic152 illustrates that discharge current increases with voltage.

The current-voltage characteristic 152 for the same magnetron plasmasystem generates a relatively low or moderate plasma density (less than10¹²-10¹³ cm⁻¹, measured close to the cathode/target surface) in alow-current regime. The plasma density in the low-current regime isrelatively low because the plasma is mainly generated by directionization of ground state atoms in the feed gas. The term “low-currentregime” is defined herein to mean the range of plasma discharge currentdensities that are less than about 0.5 A/cm² for typical sputteringvoltages of between about −300V to −1000V. The power density is lessthan about 250 W/cm² for plasmas in the low-current regime. Sputteringwith discharge voltages greater than −800V can be undesirable becausesuch high voltages can increase the probability of arcing and can tendto create sputtered films having relatively poor film quality.

The current-voltage characteristic 154 represents actual data for aplasma generated by the pulsed power supply 102 in the plasma sputteringsystem 100 of FIG. 1. The current-voltage characteristic 154 illustratesthat the discharge current is about 140 A (˜1.8 A/cm²) at a voltage ofabout −500V. The discharge current is about 220 A (˜2.7 A/cm²) when thevoltage is about −575V. The data depends on various parameters, such asthe magnitude and geometry of the magnetic field, chamber pressure, gasflow rate, pumping speed, and the design of the pulsed power supply 102.For certain operating conditions, the discharge current can exceed 375 Awith a discharge voltage of only −500V.

The voltage-current characteristic 154 is in a high-current regime. Thecurrent-voltage characteristic 154 generates a relatively high plasmadensity (greater than 10¹²-10¹³ cm⁻³) in the high-current regime. Theterm “high-current regime” is defined herein to mean the range of plasmadischarge currents that are greater than about 0.5 A/cm² for typicalsputtering voltages of between about −300V to −1000V. The power densityis greater than about 250 W/cm² for plasmas in the high-current regime.The voltage-current characteristic 154 generates high-density plasmasthat can be used for high-deposition rate magnetron sputtering.

Some known magnetron systems operate within the high-current regime forvery short periods of time. However, these known magnetron systemscannot sustain and control operation in the high-current regime for longenough periods of time to perform any useful plasma processing. Thepulsed power supply 102 of the present invention is designed to generatewaveforms that create and sustain the high-density plasma withcurrent-voltage characteristics in the high-current regime.

FIG. 3 is measured data 200 of a particular voltage pulse 202 generatedby the pulsed power supply 102 of FIG. 1 operating in a low-powervoltage mode. The pulsed power supply 102 produces a weakly-ionizedplasma having a low or moderate plasma density (less than 10¹² 10¹³cm⁻³) that is typical of known plasma processing systems. The pulsedpower supply 102 is operating in a low-power mode throughout theduration of the voltage pulse 202. The pulsed power supply 102 suppliesenergy to the plasma at a relatively slow rate in the low-power mode.The energy supplied by the pulsed power supply 102 in the low-power modegenerates a weakly-ionized plasma by direct ionization of the groundstate atoms in the feed gas. The weakly-ionized plasma corresponds to aplasma generated by a conventional DC magnetron.

The pulsed power supply 102 can be programmed to generate voltage pulseshaving various shapes. The desired voltage pulse of FIG. 3 is a squarewave voltage pulse as shown by the dotted line 203. However, the actualvoltage pulse 202 generated by the pulsed power supply 102 is notperfectly square, but instead includes low frequency oscillations thatare inherent to the power supply 102. Some of these low frequencyoscillations can be on the order of 50V or more. In addition, thevoltage pulse 202 has an initial value 204 of about −115V that is causedby the charge accumulation on the cathode assembly 116 for a particularrepetition rate.

The voltage pulse 202 includes an ignition stage 205 that ischaracterized by a voltage 206 having a magnitude and a rise time thatis sufficient to ignite a plasma from a feed gas. The magnitude of thevoltage pulse 202 rises to about 550V in the ignition stage 205.However, the voltage of the first pulse that initially ignites theplasma can be as high as −1500V. The ignition of the plasma is depictedas a rise in a discharge current 208 through the plasma. The duration ofthe ignition stage 205 is generally less than about 150 μsec. After theignition stage 205, the discharge current 208 continues to rise even asthe voltage 210 decreases.

The rise in the discharge current 208 is caused at least in part by theinteraction of the pulsed power supply 102 with the developing plasma.The impedance of the plasma decreases as the current density in theplasma increases. The pulsed power supply 102 attempts to maintain aconstant voltage, but the voltage decreases due to the changing plasmaresistive load. The peak discharge current 212 is less than about 50 Awith a voltage 214 that is about −450V. The power 216 that is present atthe peak discharge current 212, which corresponds to a momentary peakdensity of the plasma, is about 23 kW.

As the voltage 218 continues to decrease, the discharge current 220 andthe plasma density also decrease. As the density of the plasmadecreases, the impedance of the plasma increases. The voltage level 222corresponds to a quasi-static discharge current 224 that issubstantially constant throughout the duration of the voltage pulse 202.This region of quasi-static discharge current 224 is caused by theplasma having a substantially constant resistive load. The term“substantially constant” when applied to discharge current is definedherein to mean a discharge current with less than a 10% variation.

After about 200 μsec the oscillations dampen as the voltage 226fluctuates between about −525V and −575V, the discharge current 228remains constant with a value of about 25 A and the power 230 is betweenabout 10-15 kW. These conditions correspond to a weakly-ionized orlow-density plasma that is typical of most plasma processing systems,such as the conditions represented by the current-voltage characteristic152 described in connection with FIG. 2. The plasma density is in therange of about 10⁸-10¹³ cm⁻³.

The total duration of the voltage pulse 202 is about 1.0 msec. The nextvoltage pulse (not shown) will typically include an ignition stage 205in order to re-ignite the plasma. However, electrons generated from thefirst pulse can still be present so the required ignition voltage willtypically be much less than the first pulse (on the order of about−600V) and the ignition will typically be much faster (on the order ofless than about 200 μsec).

FIG. 4 is measured data 250 of a multi-stage voltage pulse 252 that isgenerated by the pulsed power supply of FIG. 1 that creates astrongly-ionized plasma according to the present invention. The measureddata 250 is from a magnetron sputtering system that includes a 10 cmdiameter NiV target with an argon feed gas at a pressure of about 10⁻³Torr. The multi-stage voltage pulse 252 generates a weakly-ionizedplasma in the low-current regime (FIG. 2) initially, and then eventuallygenerates a strongly-ionized or high-density plasma in the high-currentregime according to the present invention. Weakly-ionized plasmas aregenerally plasmas having plasma densities that are less than about10¹²-10 ¹³ cm⁻³ and strongly-ionized plasmas are generally plasmashaving plasma densities that are greater than about 10¹²-10 ¹³ cm⁻³. Themulti-stage voltage pulse 252 is presented to illustrate the presentinvention. One skilled in the art will appreciate that there arenumerous variations of the exact shape of the multi-stage pulseaccording to the present invention.

The multi-stage voltage pulse 252 is a single voltage pulse havingmultiple stages as illustrated by the dotted line 253. An ignition stage254 of the voltage pulse 252 corresponds to a voltage 256 having amagnitude (on the order of about −600V) and a rise time (on the order ofabout 4V/μsec) that is sufficient to ignite an initial plasma from afeed gas. The initial plasma is typically ignited in less than 200 μsec.

A first low-power stage 258 of the voltage pulse 252 has a peak voltage260 that corresponds to a discharge current 261 in the developinginitial plasma. In some embodiments, the ignition stage 254 isintegrated into the first low-power stage 258 such that the plasma isignited during the first low-power stage 258. The peak voltage 260 isabout −600V and can range from −300V to −1000V, the correspondingdischarge current 261 is about 20 A, and the corresponding power isabout 12 kW. In the first low-power stage 258, the pulsed power supply102 (FIG. 1) is operating in the low-power mode. In the low power mode,the pulsed power supply 102 supplies energy to the initial plasma at arelatively slow rate. The slow rate of energy supplied to the initialplasma in the low-power mode maintains the plasma in a weakly-ionizedcondition.

The weakly-ionized or pre-ionized condition corresponds to an initialplasma having a relatively low (typically less than 10¹²-10¹³ cm⁻³)plasma density. As the density of the initial plasma grows, the voltage262 decreases by about 50V as the current 261 continues to rise to about30 A before remaining substantially constant for about 200 μsec. Thedischarge current 261 rises as the voltage 262 decreases because of thechanging impedance of the plasma. As the plasma density changes, theimpedance of the plasma and thus the load seen by the pulsed powersupply 102 also changes. In addition, the initial plasma can draw energyfrom the pulsed power supply 102 at a rate that is faster than theresponse time of the pulsed power supply 102 thereby causing the voltage262 to decrease.

The impedance of the plasma decreases when the number of ions andelectrons in the plasma increases as the current density in the initialplasma increases. The increase in the number of ions and electronsdecreases the value of the plasma load. The pulsed power supply 102attempts to maintain a constant voltage. However, the voltage 262continues to decrease, at least in part, because of the changing plasmaload. The substantially constant discharge current corresponds to aconventional DC magnetron discharge current as discussed in connectionwith current-voltage characteristic 152 of FIG. 2. The initial plasmacan correspond to a plasma that is in a steady state or a quasi-steadystate condition.

The peak plasma density can be controlled by controlling the slope ofthe rise time of the voltage pulse 252. In a first transient stage 264of the voltage pulse 252, the voltage increase is characterized by arelatively slow rise time (on the order of about 2.8V/μsec) that issufficient to only moderately increase the plasma density. The plasmadensity increases moderately because the magnitude and the rise time ofthe voltage 266 in the first transient stage 264 is not sufficient toenergize the electrons in the plasma to significantly increase anelectron energy distribution in the plasma. An increase in the electronenergy distribution in the plasma can generate ionizationalinstabilities that rapidly increase the ionization rate of the plasma.The electron energy distribution and the ionizational instabilities arediscussed in more detail with respect to generating a strongly-ionizedplasma according to the invention.

The moderate increase in the plasma density will result in acurrent-voltage characteristic that is similar to the current-voltagecharacteristic 152 of a conventional DC magnetron that was described inconnection with FIG. 2. The voltage 266 increases by about 50V to avoltage peak 268 of about −650V. The discharge current 270 increases byabout 20 A to about 50 A and the power increases to about 30 kW. Thepulsed power supply 102 is still operating in the low-power mode duringthe first transient stage 264.

In a second low-power stage 272 of the voltage pulse 252, the voltage274 increases slowly by about 40V. The slow voltage increase ischaracterized by a discharge current 276 that remains substantiallyconstant for about 350 μsec. The plasma can be substantially in a steadystate or a quasi-steady state condition corresponding to thecurrent-voltage characteristic 152 of FIG. 2 during the second low-powerstage 272. The plasma density in the second low-power stage 272 isgreater than the plasma density in the first low-power stage 258, but isstill only weakly-ionized. The pulsed power supply 102 is operating inthe low-power mode.

In a second transient stage 278 of the voltage pulse 252, the pulsedpower supply 102 operates in the high-power mode. In this secondtransient stage 278, the voltage 280 increases sharply compared with thefirst transient stage 264. The rise time of the voltage 280 is greaterthan about 0.5V/μsec. The voltage increase is about 60V to the peakvoltage. The relatively fast rise time (on the order of about 5V/μsec)of the voltage 280 and the corresponding energy supplied by the pulsedpower supply 102 shifts the electron energy distribution in theweakly-ionized plasma to higher energies. The higher energy electronsrapidly ionize the atoms in the plasma and create ionizationalinstability in the plasma that drives the weakly-ionized plasma to anon-steady state condition or a transient state. In a non-steady state,the Boltzman, Maxwell, and Saha distributions can be modified. The rapidincrease in ionization of the atoms in the plasma results in a rapidincrease in electron density and a formation of the strongly-ionizedplasma that is characterized by a significant rise in the dischargecurrent 282. The discharge current 282 rises to about 250 A at anon-linear rate for about 250 μsec.

One mechanism that contributes to a sharp increase in the electronenergy distribution is known as diocotron instability. Diocotroninstability is a wave phenomena that relates to the behavior of electrondensity gradients in the presence of electric and magnetic fields.Electron electrostatic waves can propagate along and across (parallel toand perpendicular to) field lines with different frequencies. Theseelectron electrostatic waves can create electron drifts in the presenceof a perpendicular electric field that are perpendicular to magneticfield lines.

Such electron drifts are inherently unstable, since any departure fromcharge neutrality in the form of charge bunching and separation (overdistances on the order of the characteristic length scale in a plasma,the Debye length) create electric fields which cause second order ExBdrifts that can exacerbate the perturbation. These instabilities arereferred to as gradient-drift and neutral-drag instabilities. A chargeperturbation associated with an electron Hall current developed bycrossed magnetic and electric fields can produce radial electron driftwaves. Drifts driven by the two density gradients (perpendicular andparallel) associated with a maximum in the radial electron densitydistribution can interact to cause the diocotron instability. Diocotroninstability is described in “Magnetron Sputtering: Basic Physics andApplication to Cylindrical Magnetrons” by John A. Thorton., J. Voc. Sci.Technol. 15(2), March/April p. 171-177, 1978.

A high-power stage 283 includes voltage oscillations 284 that havepeak-to-peak amplitudes that are on the order of about 50V. These “sawtooth” voltage oscillations 284 may be caused by the electron densityforming a soliton waveform or having another non-linear mechanism, suchas diocotron instability discussed above, that increases the electrondensity as indicated by the increasing discharge current 286. Thesoliton waveform or other non-linear mechanism may also help to sustainthe high-density plasma throughout the duration of the voltage pulse252. Soliton waveforms, in particular, have relatively long lifetimes.

The discharge current 286 increases non-linearly through the high-powerstage 283 until a condition corresponding to the voltage-currentcharacteristic 154 of FIG. 2 is reached. This condition corresponds tothe point in which the pulsed power supply 102 is supplying an adequateamount of continuous power to sustain the strongly-ionized plasma at aconstant rate as illustrated by a substantially constant dischargecurrent 287. The peak discharge current 288 in the high-power stage 283is about 250 A at a voltage 290 of about −750V. The corresponding peakpower 292 is about 190 kW.

The voltage pulse 252 is terminated at about 1.24 msec. The cathodeassembly 116 remains negatively biased at about −300V after thetermination of the voltage pulse 252. The plasma then rapidly decays asindicated by the rapidly decreasing discharge current 294.

The high-power stage 283 of the voltage pulse is sufficient to drive theplasma from a non-steady state in the second transient stage 278 to astrongly-ionized state corresponding to the voltage-currentcharacteristic 154 of FIG. 2. The pulsed power supply 102 must supply asufficient amount of uninterrupted power to continuously drive theinitial plasma in the weakly-ionized state (in the second low-powerstage 272) through the transient non-steady state (in the secondtransient stage 278) to the strongly-ionized state (in the high-powerstage 283). The rise time of the voltage 280 in the second transientstage 278 is chosen to be sharp enough to shift the electron energydistribution of the initial plasma to higher energy levels to generateionizational instabilities that creates many excited and ionized atoms.The rise time of the voltage 280 is greater than about 0.5V/μsec.

The magnitude of the voltage 280 in the second transient stage 278 ischosen to generate a strong enough electric field between the target 118and the anode 124 (FIG. 1) to shift the electron energy distribution tohigh energies. The higher electron energies create excitation,ionization, and recombination processes that transition the state of theweakly-ionized plasma to the strongly-ionized state. The transientnon-steady state plasma state exists for a time period during the secondtransient stage 278. The transient state results from plasmainstabilities that occur because of mechanisms, such as increasingelectron temperature caused by ExB Hall currents. Some of these plasmainstabilities are discussed herein.

The strong electric field generated by the voltage 280 between thetarget 118 and the anode 124 (FIG. 1) causes several ionizationprocesses. The strong electric field causes some direct ionization ofground state atoms in the weakly-ionized plasma. There are many groundstate atoms in the weakly-ionized plasma because of its relatively lowlevel of ionization. In addition, the strong electric field heatselectrons initiating several other different type of ionization process,such as electron impact, Penning ionization, and associative ionization.Plasma radiation can also assist in the formation and maintenance of thehigh current discharge. The direct and other ionization processes of theground state atoms in the weakly-ionized plasma significantly increasethe rate at which a strongly-ionized plasma is formed.

In one embodiment, the ionization process is a multi-stage ionizationprocess. The multi-stage voltage pulse 252 initially raises the energyof the ground state atoms in the weakly-ionized plasma to a level wherethe atoms are excited. For example, argon atoms require an energy ofabout 11.55 eV to become excited. The magnitude and rise time of thevoltage 280 is then chosen to create a strong electric field thationizes the exited atoms. Excited atoms ionize at a much higher ratethan neutral atoms. For example, Argon excited atoms only require about4 eV of energy to ionize while neutral atoms require about 15.76 eV ofenergy to ionize. The multi-step ionization process is described inco-pending U.S. patent application Ser. No. 10/249,844, entitledHigh-Density Plasma Source using Excited Atoms which is assigned to thepresent assignee. The entire disclosure of U.S. patent application Ser.No. 10/249,844 is incorporated herein by reference.

The multi-step ionization process can be described as follows:Ar+e⁻→Ar*+e⁻Ar*+e⁻→Ar⁺+2e⁻

-   -   where Ar represents a neutral argon atom in the initial plasma,        e⁻ represents an ionizing electron generated in response to an        electric field, and Ar* represents an excited argon atom in the        initial plasma. The collision between the excited argon atom and        the ionizing electron results in the formation of an argon ion        (Ar⁺) and two electrons.

In one embodiment, ions in the developing plasma strike the target 118causing secondary electron emission. These secondary electrons interactwith neutral or excited atoms in the developing plasma. The interactionof the secondary electrons with the neutral or excited atoms furtherincreases the density of ions in the developing plasma as the feed gas108 is replenished. Thus, the excited atoms tend to more rapidly ionizenear the target surface 133 (FIG. 1) than the neutral argon atoms. Asthe density of the excited atoms in the plasma increases, the efficiencyof the ionization process rapidly increases. The increased efficiencycan result in an avalanche-like increase in the density of the plasmathereby creating a strongly-ionized plasma.

In one embodiment, the magnet assembly 130 generates a magnetic field132 proximate to the target 118 that is sufficient to generate anelectron ExB Hall current 135 (FIG. 1) which causes the electron densityin the plasma to form a soliton or other non-linear waveform thatincreases at least one of the density and lifetime of the plasma aspreviously discussed. In some embodiments, the strength of the magneticfield 132 required to cause the electron density in the plasma to formsuch a soliton or non-linear waveform is in the range of fifty to tenthousand gauss.

An electron ExB Hall current 135 is generated when the voltage pulse 252applied between the target 118 and the anode 124 generates primaryelectrons and secondary electrons that move in a substantially circularmotion proximate to the target 118 according to crossed electric andmagnetic fields. The magnitude of the electron ExB Hall current 135 isproportional to the magnitude of the discharge current in the plasma. Insome embodiments, the electron ExB Hall current 135 is approximately inthe range of three to ten times the magnitude of the discharge current.

The electron ExB Hall current 135 defines a substantially circular shapewhen the plasma density is relatively low. The substantially circularelectron ExB Hall current 135 tends to form a more complex shape as thecurrent density of the plasma increases. The shape is more complexbecause of the electron ExB Hall current 135 generates its own magneticfield that interacts with the magnetic field generated by the magnetassembly 130 and the electric field generated by the voltage pulse 252.In some embodiments, the electron ExB Hall current 135 becomes cycloidalshape as the current density of the plasma increases.

The electron density in the plasma can form a soliton or othernon-linear waveforms when the small voltage oscillations 284 create apulsing electric field that interacts with the electron ExB Hall current135. The small voltage oscillations 284 tend to create oscillations inthe plasma density that increase the density and lifetime of the plasma.The increase in plasma density shown in FIG. 4 in the time periodbetween about 900 μsec and 1.2 msec can be the result of the electrondensity forming a soliton or other non-linear waveform. In this timeperiod, the voltage is only slightly increasing with time, but thedischarge current 286 increases at a much more rapid rate.

In one embodiment, the electron density increases in an avalanche-likemanner because of electron overheating instability. Electron overheatinginstabilities can occur when heat is exchanged between the electrons inthe plasma, the feed gas, and the walls of the chamber. For example,electron overheating instabilities can be caused when electrons in aweakly-ionized plasma are heated by an external field and then loseenergy in elastic collisions with atoms in the feed gas. The elasticcollisions with the atoms in the feed gas raise the temperature andlower the density of the feed gas. The decrease in the density of thegas results in an increase in the electron temperature because thefrequency of elastic collisions in the feed gas decreases. The increasein the electron temperature again enhances the heating of the gas. Theelectron heating effect develops in an avalanche-like manner and candrive the weakly-ionized plasma into the transient non-steady state.

FIG. 5A-FIG. 5C are measured data 300, 300″, and 300′″ of otherillustrative multi-stage voltage pulses 302, 302′, and 302″ generated bythe pulsed power supply 102 of FIG. 1. The desired pulse shapesrequested from the pulsed power supply 102 are superimposed in dottedlines 304, 304′, and 304″ onto each of the respective multi-stagevoltage pulses 302, 302′, and 302″. The voltage pulses 302, 302′, and302″ are generated for a magnetron sputtering source having a 10 cmdiameter copper target and operating with argon feed gas at a chamberpressure of approximately 10⁻¹ Torr. The repetition rate of the voltagepulses is 40 Hz.

The voltage pulse 302 illustrated in FIG. 5A is a two-stage voltagepulse 302 having a transient region included in both the low-power stageand the high-power stage of the pulse. A low-power stage 306 of thevoltage pulse 302 including the first transient region is sufficient toignite an initial plasma and eventually sustain a weakly-ionized plasma.The duration of the low-power stage 306 of the voltage pulse 302 isabout 1.0 msec.

The relatively fast rise time (on the order of about 6.25V/μsec) of thevoltage during the first transient region in the low-power stage 306 issufficient to shift the electron energy distribution of the initialplasma to higher energies to generate ionizational instability thatdrives the initial plasma into a transient non-steady state condition.The rise time of the voltage should be greater than about 0.5V/μsec aspreviously discussed. However, since the pulsed power supply 102 isoperating in a low-power mode during the low-power stage 306 of thevoltage pulse 302, it does not supply a sufficient amount ofuninterrupted power to continuously drive the initial plasma from thetransient non-steady state to a strongly-ionized state corresponding tothe current-voltage characteristic 154 of FIG. 2. Since there isinsufficient energy stored in the pulsed power supply 102 in thelow-power mode to create conditions that can sustain a strongly-ionizedplasma, the plasma density oscillates and eventually the transientnon-steady state of the plasma becomes weakly-ionized corresponding tothe current-voltage characteristic 152 of FIG. 2.

The low-power stage 306 of the voltage pulse 302 includes relativelylarge voltage oscillations 308. The voltage oscillations 308 dampen whenthe initial plasma reaches the weakly-ionized condition corresponding tothe current-voltage characteristic 152 of FIG. 2. The weakly-ionizedplasma is characterized by the substantially constant discharge current312. The voltage oscillations 308 occur because the pulsed power supply102 does not supply enough energy in the low-power mode to drive thetransient plasma into the strongly-ionized state that corresponds to thehigh-current regime illustrated by the current-voltage characteristic154 of FIG. 2. Consequently, the discharge current 310 oscillates as theplasma rapidly expands and contracts. The rapidly expanding andcontracting plasma causes the output voltage 308 to oscillate inresponse to the changing plasma load. The rapidly expanding andcontracting plasma also prevents the electron density in the plasma fromforming a soliton or other non-linear waveform that can increase theplasma density.

The average power 314 during the generation of the initial plasma isless than about 50 kW. The voltage 316 and the discharge current 318 aresubstantially constant after about 500 μsec, which corresponds to aplasma in a weakly-ionized condition.

A high-power stage 320 of the voltage pulse 302 includes a secondtransient region 321. The voltage increases by about 30V in the secondtransient region 321. The pulsed power supply 102 generates thehigh-power stage 320 of the voltage pulse 304 at about 1.1 msec. Thevoltage in the second transient region 321 has a magnitude and a risetime (on the order of about 5V/μsec) that is sufficient to drive theweakly-ionized plasma into a transient non-steady state. The rise timeof the voltage is greater than about 0.5V/μsec. In the high-power stage320, the pulsed power supply 102 is operating in the high-power mode andsupplies a sufficient amount of uninterrupted power to drive theweakly-ionized plasma from the transient non-steady state to astrongly-ionized state corresponding to the current-voltagecharacteristic 154 of FIG. 2.

Voltage oscillations 322 occur for about 300 μsec. The voltageoscillations 322 create current oscillations 324 in the transientplasma. The voltage oscillations 322 are caused, at least in part, bythe changing resistive load in the plasma. The pulsed power supply 102attempts to maintain a constant voltage and a constant dischargecurrent, but the transient plasma exhibits a rapidly changing resistiveload.

The voltage oscillations 322 can also be caused by ionizationalinstabilities in the plasma as previously discussed. Ionizationalinstabilities can occur when the degree of ionization in the plasmachanges because of varying magnitudes of the crossed electric andmagnetic fields. The degree of ionization can grow exponentially as theionizational instability develops. The exponential growth in ionizationmay be a consequence of electron gas overheating as a result ofdeveloping electron Hall currents. The exponential growth in ionizationdramatically increases the discharge current.

The voltage oscillations 322 are minimized after about 1.5 msec. Theminimum voltage oscillations 323 can create a pulsing electric fieldthat interacts with the electron ExB Hall current 135 (FIG. 1) togenerate oscillations in the plasma density that increase the densityand lifetime of the plasma. The plasma is in the high-current regimecorresponding to the current-voltage characteristic 154 of FIG. 2 inwhich the pulsed power supply 102 supplies an adequate amount of energyto increase the density of the plasma non-linearly to thestrongly-ionized state. The average voltage 326 is substantiallyconstant while the current 328 increases nonlinearly with insignificantoscillations.

After the voltage oscillations 322, the average voltage 326 remainslower than the voltage 316 present during the low-power stage 306 of thevoltage pulse 304. The discharge current 324 rises to a peak current330. After about 2.0 msec the average voltage 326 is about −500V, thedischarge current 330 is almost 300 A and the power 332 is about 150 kW.These conditions correspond to a strongly-ionized plasma in thehigh-current regime.

The pulsed power supply 102 supplies power to the transient plasmaduring the high-power stage 320 at a relatively slow rate. Thisrelatively slow rate corresponds to a relatively slow rate of increasein the discharge current 328 over a time period of about 1.0 msec. Inone embodiment of the invention, the pulsed power supply 102 supplieshigh-power to the plasma relatively quickly thereby increasing thedensity of the plasma more rapidly. The density of the plasma can alsobe increased by increasing the pressure inside the plasma chamber.

FIG. 5A illustrates that in order to sustain a strongly-ionized plasmain the high-current regime corresponding to the current-voltagecharacteristic 154 of FIG. 2 at least two conditions must be satisfied.The first condition is that the rise time of a voltage in a transientregion must be sufficient to shift the electron energy distribution ofthe initial plasma to higher energies to generate ionizationalinstability that drives the plasma into a transient non-steady statecondition. The second condition is that the pulsed power supply mustsupply a sufficient amount of uninterrupted power to drive the plasmafrom the transient non-steady state to a strongly-ionized statecorresponding to the current-voltage characteristic 154 of FIG. 2.

In the low-power stage 306, the voltage in the first transient regionhas a sufficient rise time to shift the electron energy distribution ofthe initial plasma to higher energies as shown by current oscillations310. However, the pulsed power supply 102 is in the low-power mode anddoes not supply a sufficient amount of uninterrupted power to drive theinitial plasma from the transient non-steady state to a strongly-ionizedstate. In the high-power stage 320, the voltage in the second transientregion 321 has a sufficient rise time to shift the electron energydistribution of the initial plasma to higher energies as shown bycurrent oscillations 324. Also, the pulsed power supply 102 (in thehigh-power mode) supplies a sufficient amount of uninterrupted power todrive the weakly-ionized plasma from the transient non-steady state to astrongly-ionized state.

FIG. 5B is measured data 300′ of another illustrative multi-stagevoltage pulse 302′ generated by the pulsed power supply 102 of FIG. 1.The voltage pulse 302′ is a three-stage voltage pulse 302′. Thelow-power stage 306′ of the voltage pulse 302′ including a firsttransient region has a rise time and magnitude that ignites an initialplasma. The low-power stage 306′ corresponds to a low-power mode of thepulsed power supply 102 and is similar to the low-power stage 306 of thevoltage pulse 302 that was described in connection with FIG. 5A.

A transient stage 340 of the three-stage voltage pulse 302′ is atransition stage where the pulsed power supply 102 transitions from thelow-power mode to the high-power mode. The duration of the transientstage 340 is about 40 μsec, but can have a duration that is in the rangeof about 10 μsec to 5,000 μsec. The discharge voltage 342 and dischargecurrent 344 both increase sharply in the transient stage 340 aspreviously discussed.

The transient stage 340 of the voltage pulse 302′ has a rise time thatshifts the electron energy distribution in the weakly-ionized plasma tohigher energies thereby causing a rapid increase in the ionization rateby driving the weakly-ionized plasma into a transient non-steady state.Plasmas can be driven into transient non-steady states by creatingplasma instabilities from the application of a strong electric field.

A high-power stage 350 of the three-stage voltage pulse 302′ is similarto the high-power stage 320 of the two-stage voltage pulse 302 that wasdescribed in connection with FIG. 5A. However, the discharge current 352increases at a much faster rate than the discharge current 328 that wasdescribed in connection with FIG. 5A. The discharge current 328increases more rapidly because the transient stage 340 of the voltagepulse 302′ supplies high power to the weakly-ionized initial plasma at arate and duration that is sufficient to more rapidly create astrongly-ionized plasma having a discharge current 352 that increasesnon-linearly.

Voltage oscillations 354 in the high-power stage 350 are sustained forabout 100 μsec. The voltage oscillations can are caused by theionizational instabilities in the plasma as described herein, such asdiocotron oscillations. The voltage oscillations 354 cause currentoscillations 356. The maximum power 358 in the third stage 350 isapproaching 200 kW, which corresponds to a maximum discharge current 360that is almost 350 A. The third stage 350 of the voltage pulse 302′ isterminated after about 1.0 msec.

FIG. 5C is measured data 300″ of another illustrative multi-stagevoltage pulse 302″ generated by the pulsed power supply 102 of FIG. 1.The voltage pulse 302″ is a three-stage voltage pulse 302″. Thelow-power stage 306″ of the voltage pulse 302″ including a firsttransient region has a rise time and magnitude that ignites an initialplasma. The low-power stage 306″ corresponds to a low-power mode of thepulsed power supply 102 and is similar to the low-power stage 306 of thevoltage pulse 302 that was described in connection with FIG. 5A and thelow-power stage 306′ of the voltage pulse 302′ that was described inconnection with FIG. 5B.

A transient stage 370 of the three-stage voltage pulse 302″ is atransition stage where the pulsed power supply 102 transitions from thelow-power mode to the high-power mode. The duration of the transientstage 370 is about 60 μsec, which is about 1.5 times longer than theduration of the transient stage 340 of the voltage pulse 302′ that wasdescribed in connection with FIG. 5B. The peak-to-peak magnitude of thevoltage 376 (˜100V) is greater than the peak-to-peak magnitude of thevoltage 346 (˜70V) of FIG. 5B. The discharge voltage 372 and dischargecurrent 374 both increase sharply in the transient stage 370 because ofthe high value of the peak-to-peak magnitude of the voltage 376.

The magnitude and rise time of the transient stage 370 is sufficient todrive the initial plasma into a non-steady state condition. Thedischarge voltage 372 and the discharge current 374 increase sharply.The peak discharge voltage 376 is about −650V, which corresponds to adischarge current 377 that is greater than about 200 A. The dischargevoltage 378 then decreases as the discharge current 374 continues toincrease.

The discharge current 374 in the transient stage 370 increases at a muchfaster rate than the discharge current 352 that was described inconnection with FIG. 5B because the peak-to-peak magnitude of thevoltage 376 is higher and the duration of the transient stage 370 islonger than in the transient stage 340 of FIG. 5B. The duration of thetransient stage 370 is long enough to supply enough uninterrupted energyto the weakly-ionized plasma to rapidly increase the rate of ionizationof the transient plasma.

A high-power stage 380 of the three-stage voltage pulse 302″ is similarto the high-power stage 350 of the three-stage voltage pulse 302′ thatwas described in connection with FIG. 5B. However, the voltage pulse302″ does not include the large voltage oscillations that were describedin connection with FIGS. 5A and 5B. The large voltage oscillations arenot present in the voltage pulse 302″ because the transient plasma isalready substantially strongly-ionized as a result of the energysupplied in the transient stage 370. Consequently, the initial plasmatransitions in a relatively short period of time from a weakly-ionizedcondition to a strongly-ionized condition.

Small voltage oscillations 384 in the voltage pulse 302″ may be causedby the electron density forming a soliton waveform or having anothernon-linear mechanism that increases the electron density as indicated bythe increasing discharge current 286. The soliton waveform or othernon-linear mechanism may also help to sustain the high-density plasmathroughout the duration of the voltage pulse 302′.

The discharge current 382 in the third stage 380 is greater than about300 A. The maximum power 386 in the third stage 380 approaches 200 kW.The third stage 380 of the voltage pulse 304″ is terminated after about1.0 msec.

FIG. 6A and FIG. 6B are measured data of multi-stage voltage pulses 400,400′ generated by the pulsed power supply 102 of FIG. 1 that illustratethe effect of pulse duration in the transient stage of the pulse on theplasma discharge current. The multi-stage voltage pulses 400, 400′ wereapplied to a standard magnetron with a 15 cm diameter copper target. Thefeed gas was argon and the chamber pressure was about 3 mTorr.

The multi-stage voltage pulse 400 shown in FIG. 6A is a three-stagevoltage pulse 402 as indicated by the dotted line 404. A low-power stage406 of the voltage pulse 402 has a magnitude and a rise time that issufficient to ignite a feed gas and generate an initial plasma. Thepulsed power supply 102 is operating in the low-power mode during thelow-power stage 406. The maximum voltage in the low-power stage 406 isabout −550V. The initial plasma develops into a weakly-ionized plasmahaving a relatively low-level of ionization corresponding to thecurrent-voltage characteristic 152 of FIG. 2. The weakly-ionized plasmacan be in a steady state corresponding to a substantially constantdischarge current 408 that is less than about 50 A.

The pulsed power supply 102 is in the high-power mode during a transientstage 410. In the transient stage 410, the voltage increases by about100V. The rise time of the voltage increase is sufficient to create astrong electric field through the weakly-ionized plasma that promotesexcitation, ionization, and recombination processes. The excitation,ionization, and recombination processes create plasma instabilities,such as ionizational instabilities, that result in voltage oscillations412. The duration of the transient stage 410 of the voltage pulse 402is, however, insufficient to shift the electron energy distribution inthe plasma to higher energies because the energy supplied by the pulsedpower supply 102 in the transient stage 410 is terminated abruptly asillustrated by the dampening discharge current 414. Consequently, thetransient plasma exhibits ionizational relaxation and eventually decaysto a weakly-ionized plasma state corresponding to a substantiallyconstant discharge current 416.

A high-power stage 418 of the voltage pulse 402 has a lower magnitudethan the transient stage 410 of the voltage pulse, but a highermagnitude than the low-power stage 406. The high-power stage 418 issufficient to maintain the weakly-ionized plasma, but cannot drive theplasma from the weakly-ionized condition to the strongly-ionizedcondition corresponding to the current-voltage characteristic 154 ofFIG. 2. This is because the transient stage 410 did not provide theconditions necessary to sufficiently shift the electron energydistribution in the weakly-ionized plasma to high enough energies tocreate ionizational instabilities in the plasma. The voltage pulse 402is terminated after about 2.25 msec.

The multi-stage voltage pulse 400′ illustrated in FIG. 6B is athree-stage voltage pulse 402′ as indicated by the dotted line 404′. Alow-power stage 406′ of the voltage pulse 402′ is similar to thelow-power stage 406 of the voltage pulse 402 that was described inconnection with FIG. 6A. The low-power stage 406′ has a magnitude and arise time that is sufficient to ignite a feed gas and generate aninitial plasma. The pulsed power supply 102 is operating in thelow-power mode during the low-power stage 406′. The maximum voltage inthe low-power stage is also about −550V. The initial plasma developsinto a weakly-ionized plasma having a relatively low-level of ionizationcorresponding to the current-voltage characteristic 152 of FIG. 2. Theweakly-ionized plasma can be in a steady state corresponding to asubstantially constant discharge current 408′ that is less than about 50A.

The transient stage 410′ of the voltage pulse 402′ creates a strongelectric field through the weakly-ionized plasma that promotesexcitation, ionization, and recombination processes. The excitation,ionization, and recombination processes create plasma instabilities,such as ionizational instabilities, that result in voltage oscillations412′. The rise time of the peaks in the oscillating voltage 412′ createinstabilities in the weakly-ionized plasma that rapidly increase theionization rate of the weakly-ionized plasma as illustrated by therapidly increasing discharge current 414′.

The duration of the transient stage 410′ of the voltage pulse 402′ issufficient to shift the electron energy distribution in the plasma tohigher energies that rapidly increase the ionization rate. The durationof the transient stage 410′ of FIG. 6B is five times more than theduration of the transient stage 410 of FIG. 6A. The discharge current420 increases nonlinearly as the average discharge voltage 422decreases. The magnitude of the discharge current can be controlled byvarying the magnitude and the duration of the transient stage 410′ ofthe voltage pulse 402′.

The high-power stage 418′ of the voltage pulse 402′ has a lowermagnitude than the transient stage 410′. The pulsed power supply 102provides a sufficient amount of energy during the high-power stage 418′to maintain the plasma in a strongly-ionized condition corresponding tothe current-voltage characteristic 154 of FIG. 2. The maximum dischargecurrent 416′ for the plasma in the strongly-ionized state is about 350A. The voltage pulse 402′ is terminated after about 2.25 msec.

FIG. 7A and FIG. 7B are measured data of multi-stage voltage pulses 430,430′ generated by the pulsed power supply 102 of FIG. 1 that show theeffect of the pulsed power supply operating mode on the plasma dischargecurrent. The multi-stage voltage pulses 430, 430′ were applied to astandard magnetron with a 15 cm diameter copper target. The feed gas wasargon and the chamber pressure was about 3 mTorr.

The multi-stage voltage pulse 430 shown in FIG. 7A is a three-stagevoltage pulse 432 as indicated by the dotted line 434. The pulsed powersupply 102 generates a low-power stage 436 of the voltage pulse 432 thathas a magnitude and a rise time that is sufficient to ignite a feed gasto generate an initial plasma. The maximum voltage in the ignition stageis about −550V. The pulsed power supply 102 is operating in thelow-power mode. The initial plasma develops into a weakly-ionized plasmahaving a relatively low-level of ionization corresponding to thecurrent-voltage characteristic 152 of FIG. 2. The weakly-ionized plasmacan be in a steady state corresponding to a substantially constantdischarge current 408′ that is less than about 50 A.

The pulsed power supply 102 generates a transient stage 440 of thevoltage pulse 432 that increases the voltage by about 150V. The risetime, amplitude and duration of the voltage in the transient stage 440of the voltage pulse 432 is sufficient to promote enough excitation,ionization, and recombination processes for the weakly-ionized plasma toexperience a high rate of ionization as illustrated by the rapidlyincreasing discharge current 442. The pulsed power supply 102 isoperating in a high-power mode during the transient stage 440.

The high-power stage 444 of the voltage pulse 432 has a lower magnitudethan the transient stage 440 but has a sufficient magnitude to maintainthe strongly-ionized plasma in the high-current regime corresponding tothe current-voltage characteristic 154 of FIG. 2. The discharge current446 for the strongly-ionized plasma is about 350 A. The pulsed powersupply 102 operates in the high-power mode during the high-power stage444 and generates enough uninterrupted energy to sustain thestrongly-ionized plasma. The voltage pulse 432 is terminated after about2.25 msec.

The multi-stage voltage pulse 430′ of FIG. 7B is a three-stage voltagepulse 432′ as indicated by the dotted line 434′. The pulsed power supplygenerates a low-power stage 436′ of the voltage pulse 432′ that issimilar to the low-power stage 436 of the voltage pulse 432 of FIG. 7A.The low-power stage 436′ of the voltage pulse 432′ has a magnitude and arise time that is sufficient to ignite a feed gas to generate an initialplasma. The pulsed power supply 102 is operating in the low-power mode.The maximum voltage in the ignition stage is about −550V. The initialplasma develops into a weakly-ionized plasma having a relativelylow-level of ionization. The weakly-ionized plasma can be in a steadystate that corresponds to a substantially constant discharge current438′ that is less than about 50 A.

The pulsed power supply 102 generates a transient stage 440′ of thevoltage pulse 432′ that increases the voltage by about 150V. Thetransient stage 440′ is similar to the transient stage 440 of FIG. 7A.The amplitude and duration of the transient stage 440′ of the voltagepulse 432′ is sufficient to promote enough excitation, ionization, andrecombination processes to rapidly increase the ionization rate of theweakly-ionized plasma as illustrated by the rapidly increasing dischargecurrent 442′. The pulsed power supply 102 is operating in a high-powermode during the transient stage 440′.

The pulsed power supply 102 generates a high-power stage 444′ thatincludes a voltage having a lower magnitude than the voltage in thesecond stage 440′. The voltage in the high-power stage 444′ decreases tobelow 500V which is insufficient to sustain a strongly-ionized plasma.Thus, the strongly-ionized plasma exhibits ionizational relaxation andeventually decays to a weakly-ionized plasma state corresponding to aquasi-stationary discharge current 449. The voltage pulse 432′ isterminated after about 2.25 msec.

FIG. 8 is measured data 450 for an exemplary single-stage voltage pulse452 generated by the pulsed power supply 102 of FIG. 1 that produces ahigh-density plasma according to the invention that is useful forhigh-deposition rate sputtering. The voltage pulse 452 is a single-stagevoltage pulse as indicated by the dotted line 453. The pulsed powersupply 102 operates in a high-power mode throughout the duration of thevoltage pulse 452.

The voltage pulse 452 includes an ignition region 454 that has amagnitude and a rise time that is sufficient to ignite a feed gas togenerate an initial plasma. The discharge current 458 increases afterthe initial plasma is ignited. The initial plasma is ignited in about100 μsec.

After ignition, the discharge current 460 and the voltage 456 bothincrease. The initial peak voltage 462 is about 900V. The voltage thenbegins to decrease. The discharge current 460 reaches an initial peakcurrent 464 corresponding to a voltage 466. The initial peak dischargecurrent 464 is about 150 A at a discharge voltage 466 of about The peakdischarge current 464 and corresponding discharge voltage 466corresponds to a power 468 that is about 120 kW. The time period fromthe ignition of the plasma to the initial peak discharge current 464 isabout 50 μsec. The initial plasma does not reach a steady statecondition but instead remains in a transient state.

The voltage pulse 452 also includes a transient region 454′ havingvoltage oscillations 467 that include rise times which are sufficient toshift the electron energy distribution in the initial plasma to higherenergies that create ionizational instabilities that cause a rapidincrease in the ionization rate as described herein. The initial plasmaremains in a transient state.

The voltage pulse 452 also includes a high-power region 454″. Thevoltage in the high-power region 454″ has a magnitude that is sufficientto sustain a strongly-ionized plasma. Small voltage oscillations 469 inthe voltage pulse 452 may be caused by the electron density forming asoliton waveform or having another non-linear mechanism that increasesthe electron density as indicated by the increasing discharge current470. The soliton waveform or other non-linear mechanism may also help tosustain the strongly-ionized plasma throughout the duration of thevoltage pulse 452.

The single-stage voltage pulse 452 includes a voltage 456 that issufficient to ignite an initial plasma, voltage oscillations 467 thatare sufficient to create ionizational instabilities in the initialplasma, and a voltage 472 that is sufficient to sustain thestrongly-ionized plasma. The pulsed power supply 102 operates in thehigh-power mode throughout the duration of the single-stage voltagepulse 452. The peak discharge current 470 in the high-density plasma isgreater than about 250 A for a discharge voltage 472 of about −500V. Thepower 474 is about 125 kW. The pulse width of the voltage pulse 452 isabout 1.0 msec.

FIG. 9 illustrates a cross-sectional view of a plasma sputteringapparatus 500 having a pulsed direct current (DC) power supply 501according to another embodiment of the invention. The plasma sputteringapparatus 500 includes a vacuum chamber 104 for containing a plasma. Thevacuum chamber 104 can be coupled to ground 105. The vacuum chamber 104is positioned in fluid communication with a vacuum pump 106 that is usedto evacuate the vacuum chamber 104 to high vacuum. The pressure insidethe vacuum chamber 104 is generally less than 10⁻¹ Torr for most plasmaoperating conditions.

The plasma sputtering apparatus 500 also includes a cathode assembly502. The cathode assembly 502 is generally in the shape of a circularring. The cathode assembly 502 includes a target 504. The target 504 isgenerally in the shape of a disk and is secured to the cathode assembly502 through a locking mechanism, such as a clamp 506. The cathodeassembly 502 is electrically connected to a first terminal 508 of thepulsed power supply 501 with an electrical transmission line 510.

In some embodiments, the plasma sputtering apparatus 500 includes anenergy storage device 503 that provides a source of energy that can becontrollably released into the plasma. The energy storage device 503 iselectrically coupled to the cathode assembly 502. In one embodiment, theenergy storage device 503 includes a capacitor bank.

A ring-shaped anode 512 is positioned in the vacuum chamber 104proximate to the cathode assembly 502 so as to form a gap 514 betweenthe anode 512 and the cathode assembly 502. The gap 514 can be betweenabout 1.0 cm and 12.0 cm wide. The gap 514 can reduce the probabilitythat an electrical breakdown condition (i.e., arcing) will develop inthe chamber 104. The gap 514 can also promote increased homogeneity ofthe plasma by controlling a gas flow through the gap. The anode 512 caninclude a plurality of feed gas injectors 516 that inject feed gas intothe gap 514. In the embodiment shown, the feed gas injectors 516 arepositioned within the anode 512. The feed gas injectors 516 are coupledto one or more feed gas sources 518. The feed gas sources can includeatomic feed gases, reactive gases, or a mixture of atomic and reactivegases. Additionally, excited atom sources (not shown) or metastable atomsources (not shown) can be coupled to the feed gas injectors 516 tosupply excited atoms or metastable atoms to the chamber 104.

The anode 512 is electrically connected to ground 105. A second terminal520 of the pulsed power supply 501 is also electrically connected toground 105. In other embodiments, the anode 512 is electricallyconnected to the second terminal 520 of the pulsed power supply 501.

The anode 512 can be integrated with or connected to a housing 521 thatsurrounds the cathode assembly 502. An outer edge 522 of the cathode 502is isolated from the housing 521 with insulators 523. The space 524between the outer edge 522 of the cathode assembly 502 and the housing521 can be filled with a dielectric.

The plasma sputtering apparatus 500 can include a magnet assembly 525that generates a magnetic field 526 proximate to the target 504. Themagnetic field 526 is less parallel to the surface of the cathodeassembly 502 at the poles of the magnets in the magnet assembly 525 andmore parallel to the surface of the cathode assembly 502 in the region527 between the poles of the magnets in the magnetic assembly 525.

The magnetic field 526 is shaped to trap and concentrate secondaryelectrons emitted from the target 504 that are proximate to the targetsurface 528. The magnetic field 526 increases the density of electronsand therefore, increases the plasma density in the region 527. Themagnetic field 526 can also induce an electron Hall current that isgenerated by the crossed electric and magnetic fields. The strength ofthe electron Hall current depends, at least in part, on the density ofthe plasma and the strength of the crossed electric and magnetic fields.Crossed electric and magnetic fields generated in the gap 514 canenhance the ionizational instability effect on the plasma as discussedherein.

The plasma sputtering apparatus 500 also includes a substrate support530 that holds a substrate 532 or other work piece. The substratesupport 530 can be electrically connected to a first terminal 534 of aRF power supply 536 with an electrical transmission line 538. A secondterminal 540 of the RF power supply 536 is coupled to ground 105. The RFpower supply 536 can be connected to the substrate support 530 through amatching unit (not shown). In one embodiment a temperature controller542 is thermally coupled to the substrate support 530. The temperaturecontroller 542 regulates the temperature of the substrate 532.

The plasma sputtering apparatus 500 can also include a cooling system544 to cool the target 504 and the cathode assembly 502. The coolingsystem 544 can be any one of numerous types of liquid or gas coolingsystem that are known in the art.

In operation, the vacuum pump 106 evacuates the chamber 104 to thedesired operating pressure. The feed gas is injected into the chamber104 from the feed gas source 518 through the gas inlet 516. The pulsedpower supply 501 applies negative voltage pulses to the cathode 502 (orpositive voltage pulses to the anode 512) that generate an electricfield 546 in the gap 514 between the cathode assembly 502 and the anode512. The magnitude and rise time of the voltage pulse are chosen suchthat the resulting electric field 546 ionizes the feed gas in the gap514, thereby igniting an initial plasma in the gap 514.

The geometry of the gap 514 can be chosen to minimize the probability ofarcing and to facilitate the generation of a very strong electric field546 with electric field lines that are perpendicular to the surface 528of the target 504 and the cathode 502. This strong electric field 546can enhance the ionizational instability in the plasma by increasing thevolume of excited atoms including metastable atoms that are generatedfrom ground state atoms in the initial plasma. The increased volume ofexited atoms can increase the density of the plasma in a non-linearmanner as previously discussed.

The plasma is maintained, in part, by secondary electron emission fromthe target 504. In embodiments including the magnet assembly 525, themagnetic field 526 confines the secondary electrons proximate to theregion 527 and, therefore, concentrates the plasma proximate to thetarget surface 528. The magnetic field 526 also induces an electron Hallcurrent proximate to the target surface 528, which further confines theplasma and can cause the electron density to form a soliton waveform orother non-linear waveform.

Ions in the plasma bombard the target surface 528 since the target 504is negatively biased. The impact caused by the ions bombarding thetarget 504 dislodges or sputters material from the target 504. Thesputtering rate generally increases as the density of the plasmaincreases.

The RF power supply 536 generates a negative RF bias voltage on thesubstrate 532 that attracts positively ionized sputtered material to thesubstrate 532. The sputtered material forms a thin film of targetmaterial on the substrate 532. The magnitude of the RF bias voltage onthe substrate 532 can be chosen to optimize parameters, such assputtering rate and adhesion of the sputtered firm to the substrate 532,and to minimize damage to the substrate 532. The temperature controller542 can regulate the temperature of the substrate 532 to avoidoverheating the substrate 532.

Although FIG. 9 illustrates a magnetron sputtering system, skilledartisans will appreciate that many other plasma systems can utilizemethods for generating high-density plasmas using ionizationalinstability according to the invention. For example, the methods forgenerating high-density plasmas using ionizational instability accordingto the invention can be used to construct a plasma thruster. The methodof generating a high-density plasma for a thruster is substantially thesame as the method described in connection with FIG. 9 except that theplasma is accelerated through an exhaust by external fields.

FIG. 10 illustrates a schematic diagram 550 of a pulsed power supply 552that can generate multi-step voltage pulses according to the presentinvention. The pulsed power supply 552 includes an input voltage 554that charges a bank of capacitors 556. A parallel bank of high-powersolid state switches 558, such as insulated gate bipolar transistors(IGBTs) is coupled to a primary coil 560 of a pulse transformer 562. Thesolid state switches 558 release energy stored in the capacitors 556 tothe primary coil 560 of the pulse transformer 562.

The pulse transformer 562 also includes a secondary coil 564. Thevoltage gain from the pulse transformer 562 is proportional to thenumber of secondary turns in the secondary coil 564. A first end 566 ofthe secondary coil 564 is coupled to an array of driving circuits 568. Asecond end 570 of the secondary coil 564 is coupled to ground 572. Thearray of driving circuits 568 provide voltage pulses across a firstoutput 574 and a second output 576. The first output 574 can be coupledto a cathode and the second output 576 can be coupled to an anode. Thepulsed power supply 552 can provide pulse power up to about 10 MW with arelatively fast rise time.

FIG. 11 illustrates a schematic diagram 600 of a pulsed power supply 602having a magnetic compression network 604 for supplying high-powerpulses. The pulsed power supply 602 generates a long pulse with a switchand applies the pulse to an input stage of a multi-stage magneticcompression network 604. Each stage of magnetic compression reduces thetime duration of the pulse, thereby increasing the power of the pulse.

The pulsed power supply 602 includes a DC supply 606, a capacitor 608,and a power-MOS solid switch 610 for providing power to the magneticcompression network 604. The magnetic compression network 604 includesfour non-linear magnetic inductors 612, 614, 616, 618 and fourcapacitors 620, 622, 624, 626. The non-linear magnetic inductors 612,614, 616, 618 behave as switches that are off when they are unsaturatedand on when they are saturated. The magnetic compression network 604also includes a transformer 628.

When the solid switch 610 is activated, the capacitor 620 begins tocharge and the voltage V1 increases. At a predetermined value of thevoltage V1, the magnetic core of the non-linear magnetic inductor 612saturates and the inductance of the non-linear magnetic inductor 612becomes low causing the non-linear magnetic inductor 612 to turn on.This results in charge transferring from the capacitor 608 to thecapacitor 620. The electric charge stored in the capacitor 620 is thentransferred through the transformer 628 to the capacitor 622 and so on.The charge that is transferred to the capacitor 626 is eventuallydischarged through a load 630. The magnetic compression network 604 cangenerate high-power pulses up to a terawatt in tens of nanoseconds witha relatively high repetition rate.

FIG. 12 illustrates a schematic diagram 650 of a pulsed power supply 652having a Blumlein generator 654 for supplying high-power pulses. Thepulsed power supply 652 having the Blumlein generator 654 can delivershort duration high voltage pulses with a fast rise time and arelatively flat top. The pulsed power supply 652 includes a high voltageDC supply 656. A first terminal 658 of the high voltage DC supply 656 iscoupled through a current-limiting inductor 660 to a dielectric material662 that is located between an inner conductor 664 and an outerconductor 666 of a coaxial cable 668. The inner conductor 664 is coupledto ground 670 through an inductance 672. The outer conductor 666 is alsocoupled to ground 670. In operation, the high voltage power supply 656slowly charges the Blumlein generator 654. A very fast high-power switch674 discharges the charge through a load 676, such as a plasma load.

FIG. 13 illustrates a schematic diagram 700 of a pulsed power supply 702having a pulse cascade generator 704 for supplying high-power pulses. Ahigh frequency power supply 706 is coupled to a transformer 708. Thetransformer 708 is coupled to a cascade of lower voltage (1 kV to 3 kV)pulse generators 710 that are connected in series. In operation, thehigh frequency power supply 706 charges capacitors 712 in each of thepulse generators 710. Switches 714 in each of the pulse generators 710close at predetermined times thereby discharging energy in thecapacitors 712. When the required output voltage appears between theterminal 716 and ground 718, the stored energy discharges through a load720, such as a plasma load.

EQUIVALENTS

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined herein.

1. A strongly-ionized plasma generator comprising: a) a chamber forconfining a feed gas; b) an anode that is positioned inside the chamber;c) a cathode assembly that is positioned adjacent to the anode insidethe chamber; and d) a pulsed power supply having an output that iselectrically connected between the anode and the cathode assembly, thepulsed power supply generating at the output a multi-stage voltage pulsecomprising: a low-power stage including a first peak voltage having amagnitude and a rise time that is sufficient to generate aweakly-ionized plasma from the feed gas; and a transient stage includinga second peak voltage having a magnitude and a rise time that issufficient to shift an electron energy distribution in theweakly-ionized plasma to higher energies that increase an ionizationrate which results in a rapid increase in electron density and aformation of a strongly-ionized plasma.
 2. The plasma generator of claim1 further comprising a magnet that generates a magnetic field proximateto the cathode assembly.
 3. The plasma generator of claim 2 wherein themagnet is movable.
 4. The plasma generator of claim 2 wherein themagnetic field generated by the magnet confines the weakly-ionized andstrongly ionized plasmas proximate to the cathode assembly.
 5. Theplasma generator of claim 2 wherein the magnetic field generated by themagnet and an electric field generated by the multi-stage voltage pulseinduces an electron Hall current that raises the temperature of theelectrons in the weakly-ionized plasma to a temperature that enhancesthe rapid increase in electron density and the formation of thestrongly-ionized plasma.
 6. The plasma generator of claim 1 wherein thefeed gas comprises at least one of excited and metastable atoms.
 7. Theplasma generator of claim 1 wherein the magnitude of the first peakvoltage is less than 1,000V.
 8. The plasma generator of claim 1 whereinthe pulsed power supply provides enough energy for the electron energydistribution in the weakly-ionized plasma to continuously shift tohigher energies until the strongly ionized plasma is formed.
 9. Theplasma generator of claim 1 further comprising an energy storage devicethat is electrically coupled to the cathode assembly, the energy storagedevice discharging energy into the weakly-ionized plasma to enhance therapid increase in electron density and the formation of thestrongly-ionized plasma.
 10. The plasma generator of claim 1 wherein theweakly-ionized plasma has a discharge current density that is less thanabout 0.5 A/cm² and a power density that is less than about 250 W/cm².11. The plasma generator of claim 1 wherein the pulsed power supplygenerates the transient stage of the multi-stage pulse at a time that isat least 150 μsec after the generation of the weakly-ionized plasma. 12.The plasma generator of claim 1 wherein the rise time of the second peakvoltage in the transient stage is greater than about 0.5V/μsec.
 13. Theplasma generator of claim 1 wherein the magnitude of the second peakvoltage is less than about 1,000V over the first peak voltage.
 14. Theplasma generator of claim 1 wherein the second peak voltage in thetransient stage forms ionizational instabilities in the weakly-ionizedplasma.
 15. The plasma generator of claim 1 wherein the transient stagegenerates diocotron oscillations in the weakly-ionized plasma.
 16. Theplasma generator of claim 1 wherein a discharge current density of thestrongly-ionized plasma is greater than about 0.5 A/cm².
 17. The plasmagenerator of claim 1 wherein the power density of the strongly-ionizedplasma is greater than 250 W/cm².
 18. The plasma generator of claim 1wherein the multi-stage voltage pulse further comprises a high-powerstage following the transient stage, the high-power stage having avoltage that is sufficient to sustain the strongly-ionized plasma. 19.The plasma generator of claim 18 wherein the voltage in the high-powerstage comprises a relatively constant average voltage.
 20. The plasmagenerator of claim 18 wherein a lifetime of the strongly-ionized plasmais greater than about 200 μsec.
 21. A method of generating astrongly-ionized plasma, the method comprising: a) supplying feed gasproximate to an anode and a cathode assembly; b) generating aweakly-ionized plasma by applying a first voltage between the anode andthe cathode assembly, the first voltage having a magnitude and a risetime that is sufficient to ignite the feed gas; and c) generating astrongly-ionized plasma from the weakly-ionized plasma by applying asecond voltage between the anode and the cathode assembly, the secondvoltage having a magnitude and a rise time that is sufficient to shiftan electron energy distribution in the weakly-ionized plasma to higherenergies that increase an ionization rate which results in a rapidincrease in electron density and a formation of the strongly-ionizedplasma.
 22. The method of claim 21 further comprising applying amagnetic field proximate to the cathode assembly.
 23. The method ofclaim 22 further comprising moving the magnetic field.
 24. The method ofclaim 22 further comprising generating an electron Hall current from anelectric field generated by the second voltage and from the magneticfield, the electron Hall current raising the temperature of theelectrons in the weakly-ionized plasma to a temperature that enhancesthe increase in electron density and the form ation of thestrongly-ionized plasma.
 25. The method of claim 21 wherein the firstand the second voltages comprise a multi-stage voltage pulse.
 26. Themethod of claim 21 further comprising applying a third voltage betweenthe anode and the cathode assembly that sustains the strongly-ionizedplasma.
 27. The method of claim 26 wherein an average value of the thirdvoltage applied between the anode and the cathode assembly is relativelyconstant.
 28. The method of claim 21 wherein a lifetime of thestrongly-ionized plasma is greater than 200 μsec.
 29. The method ofclaim 21 wherein the weakly-ionized plasma is in a steady statecondition before the application of the second voltage.
 30. The methodof claim 21 wherein the weakly-ionized plasma is in a quasi-steady statecondition before the application of the second voltage.
 31. The methodof claim 21 further comprising discharging energy from an energy storagedevice into the weakly-ionized plasma to enhance the rapid increase inelectron density and the formation of a strongly-ionized plasma.
 32. Themethod of claim 21 wherein the magnitude and the rise time of the secondvoltage are sufficient to generate ionizational instabilities in theweakly-ionized plasma that enhance the ionization rate resulting in arapid increase in electron density and the formation of thestrongly-ionized plasma.
 33. The method of claim 32 wherein theionizational instabilities comprise diocotron instabilities.
 34. Themethod of claim 21 wherein the magnitude of the first voltage is lessthan 1,000V.
 35. The method of claim 21 wherein the rise time of thesecond voltage applied between the anode and the cathode assembly isgreater than about 0.5V/μsec.
 36. The method of claim 21 wherein themagnitude of the second voltage is less than about 1,000V over the firstvoltage.
 37. The method of claim 21 wherein the weakly-ionized plasmahas a discharge current density that is less than about 0.5 A/cm² and apower density that is less than about 250 W/cm².
 38. A method ofgenerating a strongly-ionized plasma, the method comprising: a)supplying feed gas proximate to an anode and a cathode assembly; and b)applying a voltage pulse between the anode and the cathode assembly, thevoltage pulse comprising: a first peak voltage having a magnitude and arise time that is sufficient to ignite an initial plasma from the feedgas; and a second peak voltage having a magnitude and a rise time thatis sufficient to shift an electron energy distribution in the initialplasma to higher energies that increase an ionization rate resulting ina rapid increase in electron density and a formation of thestrongly-ionized plasma that is sustained for greater than 200 μsec. 39.The method of claim 38 further comprising applying a magnetic fieldproximate to the cathode assembly.
 40. The method of claim 39 furthercomprising moving the magnetic field.
 41. The method of claim 39 furthercomprising generating an electron Hall current from an electric fieldgenerated by the voltage pulse and from the magnetic field, the electronHall current raising the temperature of the electrons in the initialplasma to a temperature that enhances the rapid increase in electrondensity and the formation of the strongly-ionized plasma.
 42. The methodof claim 38 wherein the voltage pulse further comprises a substantiallyconstant voltage that sustains the strongly-ionized plasma.
 43. Themethod of claim 38 wherein a duration of the voltage pulse is greaterthan 200 μsec.
 44. The method of claim 38 wherein the magnitude of thefirst peak voltage is less than 1,000V.
 45. The method of claim 38wherein the magnitude of the second peak voltage is less than about1,000V over the first peak voltage.
 46. The method of claim 38 whereinthe second peak voltage has a magnitude and a rise time that aresufficient to generate ionizational instabilities that enhance theionization rate resulting in the rapid increase in electron density andthe formation of the strongly-ionized plasma.
 47. An apparatus forgenerating a strongly-ionized plasma, the apparatus comprising: a) meansfor supplying feed gas proximate to an anode and a cathode assembly; b)means for generating a weakly-ionized plasma from the feed gas; c) meansfor shifting an electron energy distribution in the weakly-ionizedplasma to higher energies that increase an ionization rate which resultsin a rapid increase in electron density and a formation of thestrongly-ionized plasma from the weakly-ionized plasma; and d) means forsustaining the strongly-ionized plasma for greater than 200 μsec.